Improvised explosive devices (IEDs) are extremely dangerous partially because they are difficult to identify. A device capable of remote and in situ monitoring to detect concealed explosives would be very beneficial for a number of defense and homeland security uses.
Since terahertz wave spectroscopy has been utilized to detect a number of chemical and explosive materials and related compounds by providing their spectral signatures in the terahertz frequency range, it may have use in defense and security applications. For example, there is interest in terahertz wave spectroscopy as a technique to sense improvised explosive devices (IEDs). However, due to severe water vapor attenuation of terahertz waves in the atmosphere, reliable sensing range of terahertz wave spectroscopy has been limited to relatively short distances. For example, even though propagation of a pulsed terahertz wave for more than 145 meters has been achieved, spectroscopic measurement with an acceptable signal-to-noise ratio and false alarm rate is limited to about 30 meters. For defense and security applications, it is desirable to increase the reliable sensing range of terahertz wave spectroscopy.
Martini et al., in “Inversionless Amplification of Coherent THz Radiation”, 1998 IEEE Sixth International Conference on Terahertz Electronics Proceedings, pages 242-245, (1998), described the utilization of the coherent nature of terahertz waves generated from a photoconductive antenna and has succeeded in building a terahertz cavity. In this design, superposition of a coherent terahertz wave and a coherent background can make fields add up before dephasing between these two waves sets in. By adding the background field, an enhancement (over 100%) based on coherent construction of the terahertz wave is realized. While enhancing the generation of terahertz waves, the enhanced terahertz waves are still subject to attenuation in the atmosphere due to water vapor as described above.
There is a need for further techniques for increasing the generation of terahertz waves and for increasing the range at which terahertz waves may be reliably sensed under a range of atmospheric conditions